Marine seismic streamer steering apparatus

ABSTRACT

Marine seismic streamer steering apparatus are described having an elongate body, at least a portion of which is positioned eccentric of a marine seismic streamer, the apparatus having stability features selected from: one or more lateral steering control surfaces providing a center of lift approximately through a vertical streamer axis; one or more buoyancy elements providing a center of buoyancy through the same or a different vertical axis approximately through the center of the streamer; and combinations thereof. The apparatus have improved stability and avoid heeling during use.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to apparatus and methods for acquiringmarine seismic data acquisition, and more particularly to marine seismicstreamer positioning apparatus and methods of making and using same.

2. Related Art

Marine seismic exploration investigates and maps the structure andcharacter of subsurface geological formations underlying a body ofwater. For large survey areas, seismic vessels tow one or more seismicsources and multiple seismic streamer cables through the water. Thestreamers may be positioned using controllable steerable birds,deflectors, steerable buoys, and the like.

Despite their wide use, some steerable birds may suffer frominstability. Steerable birds comprising a body eccentric to the streamerwith control surfaces attached to the eccentric body, unless veryaccurately ballasted to be perfectly neutrally buoyant, may tend to heel(list to one side) during use. This heeling will cause instability inthe sense that a desired side force will always result in a verticalforce component as well. Another factor of instability is the fact thatthe wings are attached to a body eccentric to the streamer and in such away that the center of the lift force is eccentric from the streameraxis. This may result in the bird having different stability propertiesdepending on the bird pulling (wing lift force acting away from thestreamer) or pushing (wing lift force acting towards the streamer) onthe streamer.

It would be an advance in the art if the stability of steerable birdscomprising an eccentric body could be improved.

SUMMARY OF THE INVENTION

In accordance with the present invention, marine seismic streamersteering apparatus are described, which overcome at least some of theinstability problems encountered in known streamer positioning devices.Apparatus of the invention may be used to collect data in towed streamermarine seismic surveys, for example 3D and 4D towed streamer marineseismic surveys.

Streamer steering apparatus of the invention comprise an elongate bodypositioned eccentric of a marine seismic streamer, the apparatus havingstability features selected from: one or more lateral steering controlsurfaces providing a center of lift approximately through the center ofthe streamer; one or more buoyancy elements providing a center ofbuoyancy approximately on a vertical axis through the center of thestreamer; and combinations thereof. In certain embodiments, the one ormore lateral steering control surfaces may provide a center of lift maybe through the center of the streamer. In certain embodiments, the oneor more buoyancy elements may provide a center of buoyancy on thevertical axis through the center of the streamer.

A first apparatus embodiment within the invention comprises an elongatebody portion, at least a portion of the body having a longitudinal axisadapted to be substantially eccentric and generally horizontal with alongitudinal axis of a seismic streamer when attached thereto, theelongate body portion having removably attached thereto one or morecontrol surfaces having shape and composition such that a center of liftof the control surfaces goes through the streamer and is substantiallyvertical. Apparatus within this embodiment include those wherein the oneor more control surfaces are independently controllable. The controlsurfaces may be wings, hydrofoils, or some other surface able to providelift in a lateral direction. At least a portion of each control surfacemay be aligned with a vertical line through the streamer. The controlsurfaces may comprise a first portion having an end connected to theeccentric body, and another end connected to a second portion at aninflection point, the second portion being substantially vertical. Theinflection point may define a vertex of an angle between the firstportion and second portion of the control surfaces. Alternatively, theinflection point may be undefined and exist somewhere on a curve betweenthe first portion and the second portion. The first and second portionsmay move independently of the first portions in some embodiments, forexample through provision of appropriate gears or other means.

A second apparatus embodiment of the invention comprises an elongatebody portion, at least a portion of the body having a longitudinal axisadapted to be substantially eccentric and generally horizontal with alongitudinal axis of a seismic streamer when attached thereto, theelongate body portion having removably attached thereto two or moregenerally vertical control surfaces such that a center of lift of thecontrol surfaces is not through the streamer, each comprising a buoyancyelement at their distal ends, the upper control surface buoyancy elementcomprising a positively buoyant element, the lower control surfacebuoyancy element comprising a negatively buoyant element (ballastelement). The two control surfaces may comprise upper and lower wings.

A third apparatus embodiment of the invention comprises an elongate bodyportion, at least a portion of the body having a longitudinal axisadapted to be substantially eccentric and generally horizontal with alongitudinal axis of a seismic streamer when attached thereto, theelongate body portion having removably attached thereto two or moregenerally vertical control surfaces such that a center of lift of thecontrol surfaces is not through the streamer, the eccentric bodyincluding a yoke extending generally horizontally toward the streamerand to which are attached a generally vertical upper extension memberand a generally vertical lower extension member, the upper verticalextension member including a positive buoyancy element at its distalend, and the lower vertical extension member including a negativebuoyancy element at its distal end, the upper and lower extensionmembers having an axis passing through the streamer. The two controlsurfaces may comprise upper and lower wings.

A fourth apparatus embodiment is a combination of the first and secondembodiments, wherein the two control surfaces each comprise a buoyancyelement at their distal ends, the upper buoyancy element comprising apositively buoyant element, the lower buoyancy element comprising anegatively buoyant element (ballast element), the control surfaceshaving shape and composition such that a center of lift of the controlsurfaces goes through the streamer and is substantially vertical. Aswith the first embodiment, apparatus within this embodiment includethose wherein the one or more control surfaces are independentlycontrollable. The control surfaces may be wings, hydrofoils, or someother surface able to provide lift in a lateral direction. At least aportion of each control surface may be aligned with a vertical linethrough the streamer. The control surfaces may comprise a first portionhaving an end connected to the eccentric body, and another end connectedto a second portion at an inflection point, the second portion beingsubstantially vertical. The inflection point may define a vertex of anangle between the first portion and second portion of the controlsurfaces. Alternatively, the inflection point may be undefined and existsomewhere on a curve between the first portion and the second portion.The first and second portions may move independently of the firstportions in some embodiments, for example through provision ofappropriate gears or other means.

A fifth apparatus embodiment within the invention comprises acombination of embodiments one and three, wherein the two verticalextensions are offset along the streamer from the portions of the upperand lower control surfaces that have shape and composition such that acenter of lift of the control surfaces goes through the streamer and issubstantially vertical.

Apparatus within the invention may have certain features in common withthe previously known streamer steering apparatus known under the tradedesignation DigiFin, such as materials of construction, dimensions ofthe eccentric body, and the like, other than the inventive featuresdescribed herein. Apparatus of the invention include designs where thecenter of buoyancy lies on a vertical axis through the center of thestreamer, or alternatively, or in addition, add sufficient rightingmoment in terms of buoyancy and weight and in such a magnitude that thiseffect out weights the hydrostatic instability caused by the center ofbuoyancy of the original device being eccentric to the streamer axis.Also presented are designs for which the center of lift goes through thevertical streamer axis even though the wings are attached to a bodyeccentric to the streamer.

The invention and its benefits will be better understood with referenceto the detailed description below and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 illustrate schematic front elevation views of three knownstreamer steering devices;

FIGS. 4-5 illustrate further schematic front elevation views of thestreamer steering device of FIG. 2, illustrating potential instabilityproblems with such a device;

FIG. 6 is a schematic perspective view of a first embodiment of astreamer steering device in accordance with the present invention;

FIG. 7 is a schematic front elevation view of the streamer steeringapparatus of FIG. 6;

FIGS. 8-10 illustrate schematic front elevation views of three streamersteering devices within the invention; and

FIG. 11 is a schematic perspective view of another embodiment of astreamer steering device in accordance with the present invention.

It is to be noted, however, that the appended drawings are not to scaleand illustrate only typical embodiments of this invention, and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details and that numerous variations ormodifications from the described embodiments may be possible.

All phrases, derivations, collocations and multiword expressions usedherein, in particular in the claims that follow, are expressly notlimited to nouns and verbs. It is apparent that meanings are not justexpressed by nouns and verbs or single words. Languages use a variety ofways to express content. The existence of inventive concepts and theways in which these are expressed varies in language-cultures. Forexample, many lexicalized compounds in Germanic languages are oftenexpressed as adjective-noun combinations, noun-preposition-nouncombinations or derivations in Romanic languages. The possibility toinclude phrases, derivations and collocations in the claims is essentialfor high-quality patents, making it possible to reduce expressions totheir conceptual content, and all possible conceptual combinations ofwords that are compatible with such content (either within a language oracross languages) are intended to be included in the used phrases.

A marine seismic streamer is an elongate cable-like structure, typicallyup to several thousand meters long, which contains arrays of seismicsensors, known as hydrophones, and associated electronic equipment alongits length, and which is used in marine seismic surveying. In order toperform a 3D marine seismic survey, a plurality of such streamers aretowed at about 5 knots behind a seismic survey vessel, which also towsone or more seismic sources, typically air guns. Acoustic signalsproduced by the seismic sources are directed down through the water intothe earth beneath, where they are reflected from the various strata. Thereflected signals are received by the hydrophones, and then digitizedand processed to build up a representation of the subsurface geology.

The horizontal positions of the streamers are typically controlled by adeflector, located at the front end or “head” of the streamer, and atail buoy, located at the back end or “tail” of the streamer. Thesedevices create tension forces on the streamer which constrain themovement of the streamer and cause it to assume a roughly linear shape.Cross currents and transient forces cause the streamer to bow andundulate, thereby introducing deviations into this desired linear shape.

The streamers are typically towed at a constant depth of approximatelyten meters, in order to facilitate the removal of undesired “ghost”reflections from the surface of the water. To keep the streamers at thisconstant depth, control devices known as “birds”, are typically attachedat various points along each streamer between the deflector and the tailbuoy, with the spacing between the birds generally varying between 200and 400 meters. The birds have hydrodynamic control surfaces, commonlyreferred to as wings, which allow the position of the streamer to becontrolled as it is towed through the water. When a bird is used fordepth control purposes only, it is possible for the bird to regularlysense its depth using an integrated pressure sensor and for a localcontroller within the bird to adjust the wing angles to maintain thestreamer near the desired depth using only a desired depth valuereceived from a central control system.

While the majority of birds used thus far have only controlled the depthof the streamers, additional benefits can be obtained by using properlycontrolled horizontally steerable birds, particularly by using the typesof horizontally and vertically steerable birds disclosed in U.S. Pat.No. 6,671,223. The benefits that can be obtained by using properlycontrolled horizontally steerable birds can include reducing horizontalout-of-position conditions that necessitate reacquiring seismic data ina particular area (i.e. in-fill shooting), reducing the chance oftangling adjacent streamers, and reducing the time required to turn theseismic acquisition vessel when ending one pass and beginning anotherpass during a 3D seismic survey.

It is estimated that horizontal out-of-position conditions reduce theefficiency of current 3D seismic survey operations by between 5 and 10%,depending on weather and current conditions. While incidents of tanglingadjacent streamers are relatively rare, when they do occur theyinvariably result in prolonged vessel downtime. The loss of efficiencyassociated with turning the seismic survey vessel will depend in largepart on the seismic survey layout, but typical estimates range from 5 to10%. Simulations have concluded that properly controlled horizontallysteerable birds can be expected to reduce these types of costs byapproximately 30%.

In order to appreciate the various features of the invention, adiscussion is presented first of some of the relevant features ofpreviously known steerable birds. In the figures, the same referencenumerals are used throughout to denote the same or similar components,except where noted.

FIG. 1 illustrates at 2 a front elevation view of a streamer steeringdevice known under the trade designation Q-FIN, from WesternGeco LLC,comprising a body 4 inline with a streamer 6, and two independentlymovable wings, 8 a, 8 b attached to body 4. The axis A-A of the wingsgoes through the center of the streamer and is generally vertical. Thehorizontal axis is indicated at H.

FIG. 2 illustrates at 102 a front elevation view of a streamer steeringapparatus known under the trade designation DigiFin, from Input/Output,Stafford, Tex., and further described in their U.S. Pat. No. 7,092,315.As explained therein, the eccentric body 10 is either clamped onto thestreamer, or connected to the streamer through special connectors toallow the eccentric body 10 to rotate about the streamer 6. Theeccentric body 10 of this streamer steering device is intended to beorientated generally horizontal relative to streamer 6. The wings 8 aand 8 b may comprise one unit or two units positioned eccentric of thestreamer 6 and are mounted to a wing support element 12 extending from aconnector on streamer 6 to eccentric body 10. A generally verticalorientation of the wing(s) is achieved through hydrostatic stability bybuoyancy in the upper part of the wing 8 a and water or weight elementsin the lower part of the wing 8 b. The wing axis C-C, being eccentricto, and in general parallel to, a vertical axis A-A going through thecenter of the streamer, is a major difference between embodiments 2 and102, and may lead to instability and heeling during use.

FIG. 3 illustrates yet another known streamer steering device 202, fromSercel. Seemingly it applies the same principles as the device knownunder the trader designation Q-FIN except that it has three controlsurfaces or wings 8 a, 8 b, and 8 c extending from the body. Whether thewings are independently moveable or not is not known to the presentinventor.

The invention focuses on methods of enhancing stability of the design ofsteering device 102 illustrated in FIG. 2. Before going into the detailof embodiments of the present invention, some weaknesses of steeringdevice 102 are explained with reference to FIGS. 4A, 4B, 5A, and 5B. Thefact that the center of buoyancy and the center of lift of streamersteering device 102 are eccentric to the streamer puts very tighttolerances on the net weight/buoyancy (buoyancy−weight) of the device.If the net buoyancy of the device is not equal to zero the bird may tiltas illustrated in FIGS. 4A and 4B. The fact that the lifting surfaces(wings) 8 a, 8 b are eccentric to streamer 6 results in a system inwhich stability depends on the direction of lift produced by the wings.In this context a push scenario (wing lift pushing towards the streamer,as illustrated schematically in FIG. 5A) is less stable than a pullscenario (wing lift pulling away from the streamer, as illustrated inFIG. 5B), wherein the heavy arrows in each figure represent thedirection of force on the streamer.

A first streamer steering apparatus embodiment 100 of the invention isillustrated in perspective view in FIG. 6, and in front elevation viewin FIG. 7. In FIG. 6 the arrow F represents the direction of flow ofwater. This embodiment addresses both points discussed herein inreference to FIGS. 4 a, 4B, 5A, and 5B. The upper and lower controlsurfaces are, in embodiment 100, divided into two or more relativelystraight segments 8 a, 8 c and 8 b, 8 d, in each case having a definiteinflection point 8 e, 8 f. In other embodiments (not illustrated) theupper and lower control surfaces may be smoothly and gradually curved,without definite inflection points. Pylons 15 and 12 may extend fromeccentric body 10 to streamer 6 and connect to streamer 6 via connectors22 as described in U.S. Pat. No. 7,092,315. In any case, a major portionof each of the upper and lower control surfaces is located along a lineA-A extending vertically through the center of streamer 6. The controlsurfaces 8 a, 8 c and 8 b, 8 d are still mounted on a portion 14 a ofbody 10 eccentric to the streamer. The fact that the majority of thecontrol area and volume is located along the line A-A extendingvertically through the center of the streamer has at least two positiveeffects. First, if localizes more of the volume along line A-A extendingthrough the center of the streamer, and hence making the bird less proneto negative effects such as tilting from un-balance in net weight (asillustrated schematically in FIGS. 4A and 4B). Second, orienting thecontrol surfaces along the line A-A extending vertically through thecenter of the streamer addresses the problem illustrated in FIGS. 5A andB. The angle of attack of the wings may be adjusted by rotating thewings about their respective axis B-B (FIG. 7) not parallel to thevertical axis extending from the center of the streamer, or the angle ofattack of the wings may be achieved by rotating the wings about axis C-C(FIG. 7) parallel but eccentric to the line A-A extending verticallythrough the center of streamer 6.

Second and third streamer steering device embodiments 200 and 300 of theinvention are illustrated in FIGS. 8 and 9, respectively. In order toreduce the negative effect of non-zero net buoyancy, more hydrostaticstability may be added as illustrated in FIG. 8. In embodiment 200 abuoyancy element 16 a is added to the tip of the upper half of wing 8 a,and a ballast weight 16 b is added to the tip of the lower half of wing8 b of embodiment 102. Further buoyancy may be added to the apparatus200 by attaching a buoyancy element 17 using an extension 14 b, and aweight 19 using an extension 14 c.

Although embodiment 200 is an improvement over embodiment 102 in termsof lateral stability, more buoyancy and weight added may again lead to alarger amount of non-zero net buoyancy, and when added eccentric to thestreamer some of the increase in lateral stability may be counteractedby a heeling moment from the non-zero net buoyancy times the distance tothe center of the streamer. Therefore, adding the extra righting momentin a way that does not risk adding a heeling moment is beneficial. Thismay be achieved by adding the buoyancy element 16 a and weight 16 balong the vertical axis A-A through the center of streamer 6 asillustrated in embodiment 300 of FIG. 9. A yoke 18 is provided havingyoke arms 18 a and 18 b as illustrated. Yoke arm 18 a supports anextension element 20 a, which in turn supports buoyancy element 16 a.Yoke arm 18 b supports an extension element 20 b, which in turn supportsweight 16 b. Extensions 20 a, 20 b may be fixed or may have some degreeof rotation; although their main function is to support the buoyancy andweight elements, they may contribute some lateral force, for example ifthey have a hydrofoil shape.

A fourth streamer steering apparatus embodiment 400 of the invention isillustrated in FIG. 10, which may be described as a combination ofembodiments 100 and 200 of FIGS. 7 and 8, respectively. Embodiment 400may or may not have the extra buoyancy element 17 and weight 19 asillustrated in FIG. 8. In embodiment 400, both the center of lift andthe center of buoyancy line on vertical line A-A through the center ofstreamer 6. Control surfaces 8 a, 8 c and 8 b, 8 d may be curved ratherthan have definite inflection points, as explained previously inreference to FIG. 7.

A fifth streamer steering apparatus embodiment of the invention isillustrated in FIG. 11, which may be envisioned as a differentcombination of the embodiments of FIGS. 7 and 8. In this embodiment, thebent wing style 8 a, 8 c and 8 b, 8 d is used, with separate extensionelements 20 a and 20 b extending away vertically from streamer 6 havingan upper buoyancy element 16 a and a lower weight element 16 b at thedistal ends of the extensions. Extensions 20 a, 20 b may be fixed or mayhave some degree of rotation; although their main function is to supportthe buoyancy and weight elements, they may contribute some lateralforce, for example if they have a hydrofoil shape.

Various control systems may be useful in the invention, and since thisis not a major feature of the invention, they are only brieflysummarized here. The most important requirement for the control systemis to prevent the streamers from tangling. This requirement becomes moreand more important as the complexity and the total value of the towedequipment increases. The trend in the industry is to put more streamerson each seismic survey vessel and to decrease the horizontal separationbetween them. To get better control of the streamers, horizontalsteering becomes necessary. If the birds are not properly controlled,horizontal steering can increase, rather than decrease, the likelihoodof tangling adjacent streamers. Localized current fluctuations candramatically influence the magnitude of the side control required toproperly position the streamers. To compensate for these localizedcurrent fluctuations, the control system may utilize a distributedprocessing control architecture and behavior-predictive model-basedcontrol logic to properly control the streamer positioning devices.

In certain embodiments of the present invention, the control system forthe apparatus may be distributed between a global control system locatedon or near the seismic survey vessel and a local control system locatedwithin or near the apparatus. The global control system is typicallyconnected to the seismic survey vessel's navigation system and obtainsestimates of system wide parameters, such as the vessel's towingdirection and velocity and current direction and velocity, from thevessel's navigation system. The global control system may monitor theactual positions of each of the birds and may be programmed with thedesired positions of or the desired minimum separations between theseismic streamers. The horizontal positions of the birds can be derived,for instance, using the types of acoustic positioning systems describedin our U.S. Pat. Nos. 4,992,990 or 5,668,775, both incorporated hereinby reference. Alternatively, or additionally, satellite-based globalpositioning system equipment can be used to determine the positions ofthe equipment. The vertical positions of the birds are typicallymonitored using pressure sensors attached to the birds, as discussedbelow.

The global control system may maintain a dynamic model of each of theseismic streamers and utilize the desired and actual positions of thebirds to regularly calculate updated desired vertical and horizontalforces the birds should impart on the seismic streamers to move themfrom their actual positions to their desired positions. Because themovement of the seismic streamer causes acoustic noise (both fromseawater flow past the bird wing structures as well as cross currentflow across the streamer skin itself), it is important that the streamermovements be restrained and kept to the minimum correction required toproperly position the streamers. Any streamer positioning device controlsystem that consistently overestimates the type of correction requiredand causes the bird to overshoot its intended position introducesundesirable noise into the seismic data being acquired by the streamer.In current systems, this type of over-correction noise is often balancedagainst the “noise” or “smearing” caused when the seismic sensors in thestreamers are displaced from their desired positions.

The global control system may calculate the desired vertical andhorizontal forces based on the behavior of each streamer and also takesinto account the behavior of the complete streamer array. Due to therelatively low sample rate and time delay associated with the horizontalposition determination system, the global control system may runposition predictor software to estimate the actual locations of each ofthe birds. The global control system may also check the data receivedfrom the vessel's navigation system and the data will be filled in if itis missing. The interface between the global control system and thelocal control system may operate with a sampling frequency of at least0.1 Hz. The global control system will typically acquire the followingparameters from the vessel's navigation system: vessel speed (m/s),vessel heading (degrees), current speed (m/s), current heading(degrees), and the location of each of the birds in the horizontal planein a vessel fixed coordinate system. Current speed and heading may alsobe estimated based on the average forces acting on the streamers by thebirds. The global control system may send the following values to thelocal bird controller: demanded vertical force, demanded horizontalforce, towing velocity, and crosscurrent velocity.

The towing velocity and crosscurrent velocity are preferably“water-referenced” values that are calculated from the vessel speed andheading values and the current speed and heading values, as well as anyrelative movement between the seismic survey vessel and the bird (suchas while the vessel is turning), to produce relative velocities of thebird with respect to the water in both the “in-line” and the“cross-line” directions. Alternatively, the global control system couldprovide the local control system with the horizontal velocity and waterin-flow angle. The force and velocity values are delivered by the globalcontrol system as separate values for each bird on each streamercontinuously during operation of the control system.

The “water-referenced” towing velocity and crosscurrent velocity couldalternatively be determined using flowmeters or other types of watervelocity sensors attached directly to the birds. Although these types ofsensors are typically quite expensive, one advantage of this type ofvelocity determination system is that the sensed in-line and cross-linevelocities will be inherently compensated for the speed and heading ofmarine currents acting on said streamer positioning device and forrelative movements between the vessel and the bird.

A communication line, which may consist of a bundle of fiber optic datatransmission cables and power transmission wires, passes along thelength of the seismic streamer and is connected to the seismic sensors,hydrophones, that are distributed along the length of the streamer, andto the birds. The birds may have a pair of independently moveable wingsthat are connected to rotatable shafts that are rotated by wing motorsand that allow the orientation of the wings with respect to the birdbody to be changed. When the shafts of the bird are not horizontal, thisrotation causes the horizontal orientation of the wings to change andthereby changes the horizontal forces that are applied to the streamerby the bird.

The motors may consist of any type of device that is capable of changingthe orientation of the wings, and they are preferably either electricmotors or hydraulic actuators. The local control system controls themovement of the wings by calculating a desired change in the angle ofthe wings and then selectively driving the motors to effectuate thischange. While separate motors for each wing may be used, it would bealso be possible to independently move the wings using a single motorand a selectively actuatable transmission mechanism.

When the bird uses two wings to produce the horizontal and verticalforces on the streamer, the required outputs of the local control systemare relatively simple, the directions and magnitudes of the wingmovements required for each of the wings, or equivalently the magnitudeand direction the motors need to be driven to produce this wingmovement. While the required outputs of the local control system forsuch a two full moving wing design is quite simple, the structure andoperation of the overall system required to coordinate control of thedevice may be relatively complicated.

Another system for controlling a horizontally steerable bird isdisclosed in the previously mentioned U.S. Pat. No. 6,671,223,incorporated herein by reference. Using this type of control system, thedesired horizontal positions and the actual horizontal positions arereceived from a remote control system and are then used by a localcontrol system within the birds to adjust the wing angles. The actualhorizontal positions of the birds may be determined every 5 to 10seconds and there may be a 5 second delay between the taking ofmeasurements and the determination of actual streamer positions. Whilethis type of system allows for more automatic adjustment of the birdwing angles, the delay period and the relatively long cycle time betweenposition measurements prevents this type of control system from rapidlyand efficiently controlling the horizontal position of the bird. A moredeterministic system for controlling this type of streamer positioningdevice may be desired.

Deterministic control systems useful in the present invention includethose described in assignee's U.S. Pat. Nos. 6,932,017 and 7,080,607,both incorporated herein by reference, and co-pending U.S. applicationSer. Nos. 11/454,352; 11/454,349 and 11/455,042, all filed Jun. 16,2006, all three of which are incorporated herein by reference. In thesemethods, wing motors and wing units are connected to wing positionindicators that sense the relative positions of the wings and providemeasurements to analog to digital conversion units which convert theanalog wing position indicator measurements into digital format andconvey these digital values to a central processor unit. Various typesof wing position indicators may be used, including resistive angle ordisplacement sensors inductive sensors, capacitive sensors, hallsensors, or magneto-restrictive sensors.

A horizontal accelerometer and a vertical accelerometer, placed at rightangles with respect to one another, may also be connected to the analogto digital conversion unit, and these accelerometers may conveymeasurements that allow the central processor unit to determine the rollangle and roll rate of the bird. An angular velocity vibrating rate gyro(rategyro) may also be used to measure the roll rate of the bird. Atemperature sensor connected to the analog to digital conversion unitmay provide temperature measurements that allow the horizontalaccelerometer and vertical accelerometer to be calibrated. A pressuresensor may also be connected to the analog to digital conversion unit toprovide the central processor unit with measurements of the waterpressure at the bird. To calculate an appropriate depth value, themeasured pressure values may be filtered to limit the disturbance fromwaves. This may be done with a weight function filter that avoids thelarge phase displacements caused by mean value filters. Instead of usingan instantaneous depth value or simply calculating an average depthvalue over a given period of time (and thereby incorporating a largephase displacement into the depth value), the control system may use adifferentially weighted pressure filtering scheme. First the pressurevalues are transformed into depth values by dividing the pressure sensorreading by the seawater density and gravitational acceleration. Thesedepth values are then filtered using a weight function filter. Typicalincremental weighting functions values range from 0.96 to 0.90 (sampleweights of 1.0, 0.9, 0.81, 0.729, etc.) and the filter will typicallyprocess depth values received over a period of at least 100 seconds.

The central processor unit may be connected to a communications bus thatallows information to be exchanged between the local control system andthe global control system over the communication line that passesthrough the streamer. The bus may, for instance, utilize neuron chipsthat communicate using a local operating network protocol to control thedata transfer. The central processor unit and associated components maycomprise a very low power microprocessor, a dual UART on-chip,12-channel, 10 bit ADC on-chip, 908×8 RAM, 16 k×16 ROM, and 50 digitalI/O channels. The software running on the central processor unit mayconsist of two units, the local control unit and the hardware controlunit. It is possible to update these program units without having toopen the bird. The on-chip memory may thus only initially contain aboot-routine that enables the loading of software units into theexternal memory via the communication bus. The external program memorywill typically be a non-volatile memory so that these program units donot have to be re-loaded after every power down. The central processorunit must be able to run the local control system software fast enoughto secure the sampling frequency needed for effective local birdcontrol. This may mean, for instance, a sample rate of 10 Hz, which maybe 10 to 100 times faster than the sample rate of the communicationsbetween the global control system and the local control system. Asdiscussed above, the central processor unit may also receive data fromsensors attached to the bird. The sensed values include bird roll angle,bird roll angular velocity (roll rate), the wing angles, and the staticpressure of the water. These values are typically delivered to thecentral processor unit at a sample rate of at least 10 Hz. The followingvalues may be transmitted from the local control system to the globalcontrol system using the communication unit: the measured roll angle,the measured roll rate, the measured wing angles, the measured waterpressure, the calculated depth, and the calculated wing forces.

The control system may have a redundant communication system to increaseits overall reliability. The bird will typically have a backupcommunications channel, such as by overlaying a backup control signal ontop of the power line current. This backup communications channel isparticularly important because in the event of loss of communications tothe bird there would otherwise be no method for instructing the bird tobring the streamer to surface so the defective communications equipmentcan be repaired or replaced.

As noted earlier, except for the inventive features related toincreasing stability of the device, the inventive streamer steeringapparatus may share many of the features of the device known under thetrade designation DigiFin, as disclosed in U.S Pat. No. 7,092,315,incorporated herein by reference for its disclosure of the commonfeatures. For example, apparatus of the invention may include twoconnectors or cuffs rotatably attached to collars affixed about theperiphery of a streamer. In these embodiments, races formed on thecollars receive the connectors and allow them to rotate freely about thestreamer. An oversized stop may be provided at the rear of the rearcollar to keep the cuffs in position as the streamer is towed.Alternatively, instead of rotating about collars encircling thestreamer, the connectors could rotate about insert sections placedin-line between two streamer sections. The insert sections wouldthemselves rotatably receive the connectors in these embodiments.

Apparatus of the invention may have front and rear pylons 12, 15 thatinclude latching hardware 22 to releasably connect the apparatus to theconnectors. The pylons extend from eccentric body 10, which may be inthe form of a hollow tube that houses electronic communication andcontrol circuits, a battery, and a drive mechanism, including a motor.Wings 8 a, 8 b may extend from opposite sides of a wing support sectionof the eccentric body between the two pylons. Each wing may be mountedon opposite ends of a single shaft or on the ends of separate shafts. Adrive mechanism inside the body rotates the single shaft (or theseparate shafts) to pivot each wing about pivot axis A-A in embodiments100 and 400 (FIGS. 6, 7 and 10) defined by the shafts, or axis C-C inembodiments 200 and 300, which are offset from the cable and do notintersect its longitudinal axis.

Control surfaces in accordance with the invention may be ballasted asexplained in the U.S. Pat. No. 7,092,315 patent so that the pivot axesof its wings remain largely vertical, although, as explained herein,unless this is carefully done some heeling will occur. The specificgravity of the control surfaces and connectors, and pylons are designedto be about the same as that of the streamer cable itself, for example,buy making one of the control surfaces heavier than the other. This canbe done, for example, by making the lower wing out of a denser materialor installing a weight, such as a lead or tungsten weight, in a voidwithin the wing. The control surfaces of apparatus within the inventionmay be solid and molded out of polyurethane, and the interior of one orboth may be hollow with a void that is empty or filled with a foammaterial, such as glass-sphere-filled polyurethane orglass-sphere-filled epoxy, to keep them lightweight without affectingtheir designed shape. With the addition of one or more features of thepresent invention, apparatus of the invention are more properlyballasted and capable of steering the streamer cable to which it isattached. Other ways of maintaining stability include addition ofbuoyancy aide 17, or float, attached to the eccentric body 10 as anappendage via an extension 14 b (FIG. 8). Float 17 lowers the specificgravity of the bird assembly. Adjusting the volume of float 17 or thelength of its extension 14 b adjusts the specific gravity of theapparatus to help maintain stability and avoid heeling. Adding flotationin this way can be used alone or in conjunction with adjusting theabsolute and relative weights of the control surfaces and buoyancyelements 16 a and 16 b in embodiments 200, 300, 400, and 500 of thepresent invention. Optionally, a weight 19, negatively buoyant, may beattached via an extension 14 c to the connector at a position on theopposite side of the streamer to right the pivot axes of the birdassembly. These ballasting means may be used to pre-adjust apparatus ofthe invention before deployment underwater. They are also hydrostatic inthat they do not depend on the speed of the tow to be effective, asexplained in the U.S. Pat. No. 7,092,315 patent. Yet another way tomaintain the pivot axis of the wings vertical is to add a rudder or anaileron controlled by an aileron controller attached to the connector onthe opposite side of the streamer from the cable-steering device, asillustrated in FIG. 6 of the U.S. Pat. No. 7,092,315 patent.Alternatively, the aileron could extend from the cable-steering devicedirectly. The aileron rotates about a generally horizontal axis Hsimilar to the wings of a cable-leveling bird and provides more or lesslift to the cable-steering assembly as a function of its pitch angle ofattack. But, in this version, the amount of lift depends on the speed ofthe streamer through the water. The aileron controller may include anorientation sensor to determine its orientation relative to vertical.

Orientation sensors, such as one or more inclinometers oraccelerometers, may be used to determine the orientation. In some cases,an inclinometer alone may be sufficient. In other cases, in which cableaccelerations are frequent and significant, multiple-axis accelerometersmay be necessary. From the orientation sensor signals, the controllermay determine the orientation of the wings. The cable is steered byadjusting the angle of attack of the control surfaces.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. In the claims, no clauses are intended to be inthe means-plus-function format allowed by 35 U.S.C. §112, paragraph 6unless “means for” is explicitly recited together with an associatedfunction. “Means for” clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures.

1. A marine seismic streamer steering apparatus comprising an elongatebody at least a portion of which is positioned eccentric of a marineseismic streamer, the apparatus having stability features selected from:one or more lateral steering control surfaces providing a center of liftapproximately through a vertical streamer axis; one or more buoyancyelements providing a center of buoyancy approximately through the sameor a different vertical axis through the center of the streamer; andcombinations thereof.
 2. The apparatus of claim 1 wherein the bodyportion eccentric to the streamer has a longitudinal axis substantiallyhorizontal to and substantially parallel to a longitudinal axis of theseismic streamer when attached thereto.
 3. The apparatus of claim 2wherein the control surfaces are removably attached to the apparatus. 4.The apparatus of claim 1 wherein one or more of the control surfaces areindependently controllable.
 5. The apparatus of claim 1 wherein thecontrol surfaces are selected from wings, hydrofoils, or other surfaceable to provide lift.
 6. The apparatus of claim 1 wherein at least aportion of each control surface is aligned with a vertical line throughthe streamer.
 7. The apparatus of claim 1 wherein the control surfacescomprise a first portion having an end connected to the eccentric bodyportion, and another end connected to a second portion at an inflectionpoint, the second portion being substantially vertical.
 8. The apparatusof claim 7 wherein the inflection point defines a vertex of an anglebetween the first portion and second portion of the control surfaces. 9.The apparatus of claim 1 wherein the control surfaces each comprise asmoothly curved body having an end connected to the eccentric bodyportion.
 10. A marine seismic streamer steering apparatus comprising anelongate body portion, at least a portion of the body having alongitudinal axis adapted to be substantially eccentric andsubstantially horizontal with a longitudinal axis of a seismic streamerwhen attached thereto, the elongate body portion having removablyattached thereto two or more substantially vertical control surfacessuch that a center of lift of the control surfaces is not through thestreamer, each comprising a buoyancy element at their distal ends, theupper control surface buoyancy element comprising a positively buoyantelement, the lower control surface buoyancy element comprising anegatively buoyant element, the eccentric body having attached theretoan additional buoyancy element positioned on a line substantiallythrough the streamer and the eccentric body, and the streamer havingattached thereto a weight element positioned substantially on a linethrough the streamer and the eccentric body.
 11. The apparatus of claim10 wherein the body portion eccentric to the streamer has a longitudinalaxis substantially horizontal and substantially parallel to alongitudinal axis of the seismic streamer when attached thereto.
 12. Amarine seismic streamer steering apparatus comprising an elongate bodyportion, at least a portion of the body having a longitudinal axissubstantially eccentric and substantially horizontal with a longitudinalaxis of a seismic streamer when attached thereto, the elongate bodyportion having removably attached thereto two or more substantiallyvertical control surfaces such that a center of lift of the controlsurfaces is not through the streamer, the eccentric body including ayoke extending substantially horizontally toward the streamer and towhich are attached a substantially vertical upper extension member and asubstantially vertical lower extension member, the upper verticalextension member including a positive buoyancy element at its distalend, and the lower vertical extension member including a negativebuoyancy element at its distal end, the upper and lower extensionmembers having an axis passing through the streamer.
 13. The apparatusof claim 12 wherein the body portion eccentric to the streamer has alongitudinal axis substantially horizontal and substantially parallel toa longitudinal axis of the seismic streamer when attached thereto.
 14. Amarine seismic streamer steering apparatus comprising an elongate bodyat least a portion of which is positioned eccentric of a marine seismicstreamer, the apparatus comprising two lateral steering control surfacesconnected to the eccentric portion providing a center of liftapproximately through a vertical streamer axis, wherein the two controlsurfaces each comprise a buoyancy element at their distal ends, theupper buoyancy element comprising a positively buoyant element, thelower buoyancy element comprising a negatively buoyant element.
 15. Theapparatus of claim 14 wherein the control surfaces are removablyattached to the apparatus.
 16. The apparatus of claim 14 wherein thecontrol surfaces comprise a first portion having an end connected to theeccentric body portion, and another end connected to a second portion atan inflection point, the second portion being substantially vertical.17. The apparatus of claim 14 wherein the control surfaces each comprisea smoothly curved body having an end connected to the eccentric bodyportion.
 18. The apparatus of claim 14 wherein the body portioneccentric to the streamer has a longitudinal axis substantiallyhorizontal to and substantially parallel to a longitudinal axis of theseismic streamer when attached thereto.
 19. A marine seismic streamersteering apparatus comprising an elongate body at least a portion ofwhich is positioned eccentric of a marine seismic streamer, theapparatus comprising two lateral steering control surfaces connected tothe eccentric body portion, each control surface having a verticalportion providing a center of lift approximately through a verticalstreamer axis, the apparatus further comprising two substantiallyvertical extensions extending from the streamer offset along thestreamer from the vertical portions of the control surfaces.
 20. Theapparatus of claim 19 wherein the body portion eccentric to the streamerhas a longitudinal axis substantially horizontal to and substantiallyparallel to a longitudinal axis of the seismic streamer when attachedthereto.